Conventional hydraulic apparatus comprising a hydraulic pump, an accumulator, a solenoid valve, an actuator (hydraulic cylinder), etc., has the following drawbacks:
(A) The hydraulic apparatus is complicated in construction and unavoidably large-sized;
(B) The slide faces allow oil leakage; and
(C) The operation of the solenoid valve at a frequency higher than several hundred cycles is not possible because of its inferior frequency characteristics.
For the above reasons, the conventional hydraulic apparatus is unsuitable for a wide range of applications, e.g., a robot, because it is incapable of satisfactorily meeting requirements such as compactness, lightness in weight, accuracy in positioning, and quickness in response.